1. Field of the Invention
This invention relates to alternators of the type that are used in vehicles to provide electrical power for running accessories and charging batteries. More particularly, this invention relates to a high-efficiency hybrid alternator in which the rotating magnetic field is provided by a rotor having permanent magnet poles and wound field poles operating in combination. The invention also relates to voltage regulators specially designed to automatically regulate the output voltage of hybrid alternators.
2. Description of Related Art
The automotive industry has been attempting to increase the efficiency of motorized vehicles, both at idle and at running speeds. The alternator design most commonly found in vehicles has been used for approximately twenty-five to thirty years and is inexpensive to produce, but exhibits very low efficiency levels, as low as 40-50%. The problem is particularly acute at low RPMs where high excitation levels in the rotor winding are required to produce the desired voltage, leading to very low efficiency.
In conjunction with the desire for higher efficiency is the need to supply alternators that have larger electrical ratings because modern vehicles have many more motors and require much more electrical power. Moreover, fuel efficiency of vehicles is closely related to the weight of the vehicle and it is desirable to decrease the weight of the alternator so as to minimize the total vehicle weight. These objectives are achieved when the efficiency of the alternator is increased.
The increased power usage in vehicles has also led to an interest in using components that operate at higher voltages than the standard 12 volts presently used in automobiles. At the same time, it is foreseen that 12 volt power will be required in such vehicles in addition to the higher voltage.
It is known to provide dual voltage alternators by providing two windings on the stator. However, when a single winding is used on the rotor, it is difficult to properly regulate the two different voltage outputs as different levels of rotor excitation current may be required for the different circuits. Single and dual voltage alternators of the type represented by the present invention may also be used in various non-engine driven applications, such as wind or water driven applications, for the efficient generation of electrical power.
Hybrid alternators significantly increase their efficiency by using permanent magnets to produce a high level of magnetic flux immediately, while the alternator is operating at low speed. Using the hybrid alternator disclosed herein, the alternator will produce full rated alternator current and voltage output at engine idling speed when installed in an automobile or other vehicle. This can be contrasted with prior art alternators that are incapable of producing their full rated output until they are turning at speeds far above their rotational speed at idle.
The full rated output of the hybrid alternator is achieved at low speed by supplementing the magnetic flux produced by the permanent magnets. The supplementing magnetic flux is produced by a rotor winding having a forward rotor winding current induced therein by a forward polarity voltage applied across the winding. This is referred to as the boosting mode or the forward polarity mode in which the wound field induced magnetic field is in the same direction as, and supplements, the permanent magnet induced magnetic field.
As the alternator RPM increases, however, the magnetic flux from the permanent magnets produces a greater output and the need for the supplementing flux from the rotor winding decreases. Ultimately, at a sufficiently high speed, all of the alternator's rated output is available solely from the permanent magnet induced magnetic field, and no additional current is needed in the rotor winding. Generally, this transition occurs at a speed well below the maximum anticipated operating speed of the alternator.
As the rotor speed exceeds this transition point, with the engine operating at a high speed, the flux from the permanent magnets is too great and must be reduced to avoid producing damaging overvoltages and overcurrents. This is accomplished by operating the hybrid alternator in the bucking mode or the reverse polarity mode in which a reverse polarity voltage is applied to the rotor winding. The reverse polarity voltage produces a reverse current in the rotor winding. The reverse current generates a magnetic flux which opposes the magnetic flux from the permanent magnets, thereby reducing the output of the alternator to maintain the desired output voltage.
The necessity for both forward and reverse rotor winding excitation current imposes certain limitations and requirements on the voltage regulator for the hybrid alternator which are not required in the case of conventional alternators. Although hybrid alternators of a low efficiency claw pole or Lundell type design are known, the existence of these limitations and requirements has not heretofore been recognized by the art even when producing voltage regulators for hybrid alternators.
A first problem is related to the inductive effects of switching the highly inductive rotor winding, particularly to transition between the forward and reverse polarity excitation modes. The problem is most acute when the alternator is lightly loaded and a battery is not connected to the alternator. In this condition, a net instantaneous negative current may be introduced onto the main power bus.
Current induced in the field winding stores significant energy in the magnetic field of the rotor winding, This energy can cause voltage spikes due to sudden load changes or when switching the voltage to drive the rotor winding. To reduce the output voltage of a hybrid alternator, the prior art has simply indicated that the reverse polarity mode should be applied to reduce or reverse the current in the field winding. However, before the current can be reversed, the previously induced magnetic field must collapse. During this collapse, the forward current originally induced in the forward polarity mode continues back up into the main power bus leading to the battery and all of the automobile accessories.
In implementing the prior art system of regulation, a bridge circuit has been used providing two state voltage pulse width modulation. This type of modulation results in negative current steps into the main power bus with the negative step amplitude equal to the magnitude of the field current. If the load current on the main power bus is less than the magnitude of the field current, a net negative current is applied to the bus. This current has no place to go because the alternator diodes prevent negative current flow into the alternator and result in a destructive voltage spike unless suppressed by the battery or a large bus capacitor.
If a battery is connected to the alternator as in the normal case, the battery can be relied upon to absorb any net negative current after the battery's other loads. Alternatively, a large capacitor can be used to absorb this energy. However, the first method cannot be relied upon as a battery may not always be present capable of absorbing the reverse current. Using a capacitor is extremely expensive, particularly when capacitors adequate for handling all the energy stored in the rotor winding are used that are temperature rated for use under the hood of an automobile.
If the battery were to be removed, without a capacitor there would be no place for the net reverse current on the main power bus to go unless a large filter capacitor is placed across the circuit where the battery connection normally exists. If moderate frequency pulse width modulation techniques are employed, this capacitor can be of reasonable value. However, for lowest costs and small physical size an aluminum electrolytic capacitor would be desirable. Aluminum electrolytic capacitors, however, are not normally designed to tolerate temperatures in excess in 105.degree. C. and thus, they could not be easily housed in the hot environment of the alternator in the vicinity of the vehicle engine.
Even if they were somewhat isolated from the hot alternator itself so as to avoid temperatures above 105.degree. C. the life of capacitors is rapidly reduced with increasing temperature. Thus, the under the hood environment would normally not permit the use of aluminum electronics. Higher temperature tantalum capacitors could be used but they are physically larger and much more expensive and are thus less attractive for a cost sensitive high volume automotive application.
Also, even if capacitors are used to absorb the switching transients, there is still a potential problem due to the large energy storage and long time constant of the field coil. For example, if the alternator speed or load should abruptly change so as to cause the alternator regulator to change the field voltage polarity from near full voltage (e.g. boost in the forward polarity mode) in one direction to significant voltage in the other direction (e.g. buck in the reverse polarity mode) a large voltage transient would tend to occur if no battery were present and the system was unloaded (except for field coil).
In this situation the initial energy in field coil would tend to go into the capacitor and the voltage would be excessive unless the capacitor were extremely large or the bus voltage were clamped.
Although only moderate sized capacitors would be required to handle the ripple current from the pulse with modulation, the capacitor would have be physically very large to be able handle the high energy in a field winding without creating an excessive voltage. Even if voltage clamps were employed to limit the capacitor voltage, the costs would be excessive, there would be continuing concerns over reliability due to the high temperature environment, and the size of the components would create a problem in the cramped environment under the hood.
A solution allowing the use of pulse width modulation techniques, even if the battery is not present, and one that does not require a large capacitor is needed.
A second, more subtle, problem is that precautions must be taken to prevent the voltage regulator that is providing the reverse current in the reverse polarity mode from being inactivated when the vehicle is turned off. At very high engine and alternator speeds, the magnetic flux from the permanent magnet is almost completely cancelled by the oppositely directed magnetic flux in the hybrid rotor winding. If the canceling flux were to be immediately turned off, e.g. by turning off an ignition switch with the alternator operating at a high rotational speed, the output voltage of the alternator would rapidly increase to damaging levels for the electrical components in a typical automobile.
The present invention incorporates an automatic interlock which powers the voltage regulator automatically and independently of the ignition system of the vehicle to prevent it from inadvertently being deactivated. The design of the automatic interlock is such that little or no current is drawn from the vehicle battery when the vehicle is off, which might tend to discharge the vehicle battery.
The preferred embodiment of the voltage regulator also incorporates transient voltage suppression in a novel way that permits certain switches (preferably FETs) needed for the purpose of switching the rotor winding between forward and reverse polarity modes to perform a second function of suppressing voltage transients that might damage the voltage regulator or other systems on the battery bus.
In view of the problems with the prior art, one object of the present invention is to provide an alternator which operates efficiently at low RPMs.
Another object of the invention is to provide an alternator which uses a permanent magnet assembly in the rotor to provide a rotating permanent magnetic field in combination with a rotating variable magnetic field generated by a rotor winding.
Still another object of the invention is to provide an alternator which weighs less than current alternators at the same output power or which produces a higher output at the same weight.
Yet another object of the present invention is to provide an efficient dual voltage alternator, preferably in which both voltages are well regulated under varying loads.
Another object of the invention is to provide a voltage regulator for a hybrid alternator that automatically interlocks to prevent the regulator from being deactivated when the alternator is in the reverse polarity mode.
Still another object of the invention is to provide a voltage regulator for a hybrid alternator which provides voltage transient suppression.
A further object of the invention is to provide a voltage regulator for a hybrid alternator that allows the alternator to operate without a battery attached and without requiring expensive capacitors or voltage clamps.
Yet another object of the invention is to provide a hybrid alternator which provides the maximum rated output voltage and current when a vehicle in which the alternator is installed is operating at idle speed.
A further object of the invention is to provide an alternator which is maximally cooled through radial cooling slots located in the stator.